Vanadia-titania metal molybdate dual catalyst bed system and process using the same for methanol oxidation to formaldehyde

ABSTRACT

A process and a catalyst reaction zone comprising one or more fixed bed reactors for oxidizing methanol in a reactant gas feed stream to formaldehyde. According to one embodiment, the process comprises introducing the reactant gas feed stream into an upstream region containing a vanadia-titania first catalyst (substantially free of a volatile MoO 3  species) under oxidizing conditions to form a partially oxidized reactant gas feed stream which is then introduced under oxidizing conditions into a downstream region containing a metal molybdate second catalyst to further oxidize any residual methanol contained therein. According to another embodiment, a fixed bed reactor comprising an upstream region and a downstream region containing the aforementioned vanadia-titania and metal molybdate catalysts, respectively, is utilized to implement the inventive process to yield a product gas stream containing formaldehyde preferably at a conversion of 85% or more and a selectivity of 90% or more.

This application is a divisional application of prior filed U.S.application Ser. No. 09/950,833, filed Sep. 13, 2001, now U.S. Pat. No.6,552,233, which claims the benefits of provisional application60/232,511, filed Sep. 14, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a catalytic reactor bed arrangement comprising,in a specified distribution, a plurality of catalysts in one or morefixed bed reactors and a process using the same for oxidation ofmethanol to formaldehyde. More particularly, the invention relates to(1) a catalytic reaction zone (e.g., one or more catalytic reactor beds)comprising, in a specified distribution, a first catalyst ofvanadia-titania and a metal molybdate second catalyst, provided in oneor more fixed bed reactors, and (2) a process using the same foroxidizing methanol or methanol containing gas streams (i.e., paper pulpmill waste streams) to formaldehyde (CH₂O).

2. Description of the Related Art

The formation of formaldehyde involves the dehydrogenation and oxidationof methanol. One approach for converting methanol to formaldehydeinvolves oxidizing methanol over a silver catalyst. See, for example,U.S. Pat. Nos. 4,080,383; 3,994,977; 3,987,107; 4,584,412; 4,343,954 and4,343,954. Typically, methanol oxidation to formaldehyde over a silvercatalyst is carried out in an oxygen lean environment. One problemassociated with silver catalyzed methanol oxidation is methanol leakage,i.e., high amounts of unconverted methanol.

Accordingly, improved processes for oxidizing methanol to formaldehydehave been developed. These processes use a methanol/air mixture (e.g., areactant gas feed stream of methanol, excess air and an inert carriergas) introduced over an iron-molybdate/molybdenum trioxide catalyst.See, for example, U.S. Pat. No. 3,983,073 (conversion of methanol toformaldehyde using Fe₂(MoO₄)₃ and MoO₃ having a molar ratio of Mo/Fefrom 1.5 to 1.7 and a degree of crystallinity of at least 90%); U.S.Pat. No. 3,978,136 (process for the conversion of methanol toformaldehyde with a MoO₃/Fe₂O₃/TiO₂ catalyst wherein the MoO₃:Fe₂O₃weight ratio is between 1:1 to 10:1 and TiO₂ is present between 1 to 90weight % of total oxides); U.S. Pat. No. 3,975,302 (a supported ironoxide and molybdenum trioxide catalyst wherein the atomic ratio of Mo/Feis from 1.5 to 5); U.S. Pat. No. 3,846,341 (a shaped and optionallysupported iron molybdate type catalyst having high mechanical strengthmade by reacting ammonium molybdate and ferric molybdate); U.S. Pat. No.3,716,497 (an optionally shaped iron molybdate type catalyst made byadmixing with NH₄ ⁺A⁻); U.S. Pat. No. 4,829,042 (high mechanicalstrength catalyst of Fe₂(MoO₄)₃ and MoO₃ together with non-sinteredFe₂O₃); U.S. Pat. No. 4,024,074 (interaction product of Fe₂(MoO₄)₃, MoO₃and bismuth oxide for catalyzing oxidation of methanol to formaldehyde);U.S. Pat. No. 4,181,629 (supported catalyst of iron oxide and molybdenumoxide on silica, alumina and the like); U.S. Pat. No. 4,421,938 (asupported catalyst of at least two oxides of Mo, Ni, Fe and the like);and U.S. Pat. No. 5,217,936 (a catalyst of a monolithic, inert carrierand oxides of molybdenum, iron and the like).

In comparison to the silver catalyzed processes,iron-molybdate/molybdenum trioxide catalyzed processes produce higheryields of formaldehyde. Iron-molybdate, Fe₂(MoO₄)₃, in combination withmolybdenum trioxide, MoO₃, constitute the metal oxide phases ofexemplary commercially available metal oxide catalysts suitable foroxidizing methanol to formaldehyde. During the oxidation of methanol toformaldehyde, the Fe₂(MoO₄)₃/MoO₃ catalyst can be generated in situ fromphysical mixtures of pure molybdenum trioxide, MoO₃, and ferric oxide,Fe₂O₃. See co-pending application designated by U.S. Provisional Ser.No. 60/081,950 of Wachs, et al. Entitled “In Situ Formation of MetalMolybdate Catalysts,” filed Apr. 15, 19098, incorporated herein byreference in its entirety. The molar ratio MoO₃/Fe₂O₃ of these catalystsmay be varied. Typically, such catalysts used in industrial andcommercial applications contain an excess of MoO₃. Thus, for example,the molar ratio MoO₃/Fe₂O₃ may vary from 1.5/1 to 12/1 or more. ExcessMoO₃ is provided to ensure that sufficient amounts of Fe₂(MoO₄)₃ areformed in situ (from the mixture of Fe₂O₃ and MoO₃) for efficientlyoxidizing methanol to formaldehyde in high yields.

Unfortunately, the use of excess MoO₃ in conjunction with Fe₂O₃ or othermetal oxides and/or metal molybdates is problematic. Oxidizing methanolto formaldehyde using a metal molybdate/molybdenum trioxide typecatalyst, e.g., Fe₂(MoO₄)₃/MoO₃, is a highly exothermic process. Theheat released during the oxidation reaction increases the catalystand/or the fixed bed reactor temperature producing “hot spots” on thecatalyst surface. These hot spots reach temperatures high enough tovolatilize the MoO₃ species present within metal molybdate/molybdenumtrioxide type catalysts. Thus, MoO₃ is sublimed from the hot spots soformed.

The sublimed MoO₃ species migrate downstream (e.g., within an exemplaryfixed bed reactor housing the catalyst) towards cooler regions of thefixed bed reactor or the like. Typically, the downstream migration ofsublimed MoO₃ species is facilitated by the incoming flow of thereactant gas feed stream containing, for example, methanol, air, and anoptional inert carrier gas fed into the inlet end of a fixed bedreactor. The migrated MoO₃ species crystallize in the cooler downstreamregions of the fixed bed reactor, for example, in the form of MoO₃crystalline needles. Over time, the needle formation accumulates andultimately obstructs the flow of the reactant gas feed stream throughthe fixed bed reactor. Thus, build up of MoO₃ crystals/needles in thedownstream region causes a substantial pressure drop in the reactant gasfeed stream flow rate as the reactant gas feed stream is directeddownstream. This pressure drop impedes the efficient oxidation ofmethanol to formaldehyde. See, for example, U.S. Pat. Nos. 3,983,073(col. 1, lines 35-52); and 4,024,074 (col. 1, lines 60-68); and U.K.Patent No. 1,463,174 (page 1, col. 2, lines 49-59) describing theaforementioned volatility problem. See also, “Fluidized bed improvesformaldehyde process,” C&EN, pp. 37-38 (Nov. 3, 1980; Popov, et al.,“Study of an Iron-Molybdenum Oxide Catalyst for the Oxidation ofMethanol to Formaldehyde,” Institute of Catalysis, Siberian Branch ofthe Academy of Sciences of the USSR, Novosibinsk, Transcript fromKiretika & Kataliz, Vol. 17, No. 2, pp. 371-377, March-April, 1976; E.M. McCarron III, et al.; “Oxy-Methoxy Compounds of Molybdenum (VI) andtheir Relationship to the Selective Oxidation of Methanol Over MolybdateCatalysts, Polyhedron, Vol. 5, No. ½, pp. 129-139 (1986); and L. Cairatiet al., “Oxidation of Methanol in a Fluidized Bed Fe₂O₃—MoO₃ SupportedSilica,” Chemistry and Uses of Molybdenum, Proceedings of the FourthInternational Conference, CLIMAX MOLYBDENUM COMPANY, H. F. Baum and P.C. H. Mitchell, Editors, pp. 402-405, Aug. 9-13, 1982.

Often, the MoO₃ needle formation that occurs in the downstream region ofthe fixed bed reactor is so excessive that the reactor must be shutdown, the needles cleaned out, and fresh catalyst charged therein. Thesesteps unnecessarily increase the time, cost, inefficiency and/orcomplexity of operating a fixed bed reactor or the like for oxidizingmethanol to formaldehyde.

The vanadia-titania (V₂O₅ supported by TiO₂) supported catalyst is acatalyst that can also selectively oxidize methanol to formaldehyde.Unfortunately, this catalyst has a disadvantage associated with its use.The vanadia-titania catalyst exhibits an extremely high catalyticactivity. Due to this high catalytic activity, this catalyst continuesto oxidize formaldehyde into carbon monoxide especially when a highconcentration of formaldehyde is available. Consequently, the yield offormaldehyde is undesirably lowered.

Accordingly, there would be an advantage to provide a catalytic reactorbed arrangement and a process using the same that substantiallyalleviates, and/or eliminates the aforementioned crystallizationproblems associated with metal molybdate catalysts containing volatileMo/MoO₃ species while simultaneously alleviating and/or eliminating theaforementioned undesirable continued oxidation of formaldehyde to carbonmonoxide associated with vanadia-titania catalysts.

Further, (1) silver catalysts, (2) supported catalysts such as thosecontaining silicon dioxide, non-sintered Fe₂O₃, bismuth interactionproducts, silica, and/or alumina, (3) catalysts containing zinc, zinccarbonates and/or indium, (4) shaped catalysts, and (5) the like areoften prohibitively expensive to use. Accordingly, there remains a needfor a catalytic bed reactor arrangement and a method using the samesuitable for cost effectively oxidizing methanol to formaldehyde whichminimizes the use of one or more of (1) silicon dioxide, (2)non-sintered Fe₂O₃, (3) interaction products of Fe₂(MoO₄)₃, and MoO₃,and bismuth, (4) silica, (5) alumina, (6) shaped catalysts forincreasing mechanical strength, (7) catalysts containingZn(CO₃)₂.3Zn(OH)₂, In(NO₃)₃.3H₂O or one or more of the compounds listedin U.S. Pat. No. 4,421,938, (8) a fibrous carrier material such assilica, or (9) the like.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a catalyticreactor bed arrangement of two or more catalysts, in a specifieddistribution, within one or more fixed bed reactors and a process usingthe same for converting methanol to formaldehyde that alleviates and/oreliminates the above-mentioned problems associated with the volatilityof MoO₃ and the undesired continued oxidation of formaldehyde to carbonmonoxide associated with vanadia-titania catalysts.

It has been surprisingly discovered that use of a substantially purevanadia-titania catalyst (e.g., essentially free of volatile MoO₃species) distributed in an upstream region of one or more fixed bedreactors together with a metal molybdena catalyst within the downstreamregion of the fixed bed reactor provides a high selectivity (e.g.,nearly 90-100%) and a high conversion % (e.g., at least 85-95%) foroxidizing methanol to formaldehyde while eliminating and/or alleviatingthe above-mentioned volatility and continued oxidation problems.

According to one aspect of the invention, oxidation of methanol toformaldehyde is achieved by the exemplary process described below. Theprocess comprises the steps of:

-   -   (a) introducing a reactant gas feed stream comprising methanol        into an inlet end of a catalyst reaction zone having said inlet        end, an upstream region, a downstream region, and an outlet end,        wherein the catalyst reaction zone comprises a fixed bed reactor        with a vanadia-titania first catalyst in the upstream region and        a metal molybdate second catalyst in the downstream region, and        wherein the upstream region is essentially free of a volatile        MoO₃ species;    -   (b) contacting and oxidizing the methanol to formaldehyde with        the vanadia-titania first catalyst to yield a partially oxidized        reactant gas feed stream containing residual methanol; and    -   (c) then contacting and oxidizing the residual methanol to        formaldehyde with the metal molybdate second catalyst to yield a        product gas stream.

According to another aspect of the invention, an exemplary catalyticreactor bed comprises a vanadia-titania catalyst in an upstream regionand a metal molybdate catalyst in a downstream region of the fixed bedreactor, respectively. The vanadia-titania catalyst must be essentiallyfree of a volatile species of MoO₃ sufficient to alleviate and/oreliminate a substantial pressure drop of the reactant gas feed stream(comprising methanol) as it flows through the fixed bed reactor. Themetal molybdate catalyst must initially be essentially free of vanadia(V₂O₅) sufficient to alleviate and/or eliminate a substantial furtheroxidation of the formaldehyde such as to carbon monoxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart depicting the process of this invention accordingto one embodiment.

FIG. 2 is a schematic of a tubular fixed bed reactor according to oneembodiment of the invention.

FIG. 3 is a schematic of a block fixed bed reactor according to anotherembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Oxidizing methanol to formaldehyde may be facilitated by the use of twoor more catalysts having a specified distribution within a catalystreaction zone. Typically, the catalyst reaction zone comprises a fixedbed reactor having an inlet end, an upstream region, a downstream regionand an outlet end. Preferably, the inlet end, the upstream region, thedownstream region and the outlet end are provided in the same order asindicated herein. For example, see FIGS. 2 and 3. The catalysts aredistributed as described in greater detail below with reference to theexemplary fixed bed reactors depicted in FIGS. 2 and 3. These fixed bedreactors are suitable for carrying out the process of the inventionincluding the exemplary process steps outlined in FIG. 1.

As used herein, the term “methanol” is intended to include pure methanolstreams, paper (pulp) mill waste gas streams containing methanol, otherwaste gas streams containing methanol, methyl mercaptan and/or mixturesthereof.

Now referring to FIG. 1, process step S₁ comprises providing at leastone fixed bed reactor (or the like). The fixed bed reactor includes acatalytic reactor bed (i.e., the upstream and downstream regions of thefixed bed reactor) comprising a plurality of catalysts in a specifieddistribution. The specified distribution comprises providing avanadia-titania first catalyst (essentially free of a volatile MoO₃species) in the upstream region of the fixed bed reactor and a metalmolybdate second catalyst (initially, essentially free of V₂O₅ species)in the downstream region of the fixed bed reactor. For the reasonspreviously described, the upstream region of the fixed bed reactor isthe region prone to substantial formation of hot spots during thecatalytic oxidation of methanol to formaldehyde. With reference to FIGS.2 and 3, typically, the upstream region 110 of depth 1 comprises fromabout ¼ to about ½ of the total depth 5 of the fixed bed reactor.Preferably, the upstream region comprises from about ¼ to about ⅜ of thetotal depth 5 of the fixed bed reactor, more preferably from about ⅓ toabout ⅜ of the total depth 5 of the fixed bed reactor.

Further, with reference FIG. 1, process step S₁ comprises providing ametal molybdate second catalyst in the downstream region of the fixedbed reactor. The downstream region of the fixed bed reactor issubstantially less prone to formation of hot spots than the upstreamregion because substantial conversion of methanol to formaldehyde (asdescribed below) has already occurred in the upstream region of thefixed bed reactor. Accordingly, only residual methanol (yet unconvertedto formaldehyde) is oxidized to formaldehyde in the downstream region.Typically, because the amount of the residual methanol is substantiallyless than the amount of the methanol in the reactant gas feed streamentering the upstream region, formation of hot spots is substantiallysuppressed in the downstream region. Accordingly, a conventional metalmolybdate type catalyst (e.g., Fe₂(MoO₄)₃ together with excess MoO₃) maybe and preferably is provided in the downstream region. As previouslynoted, because hot spot formation is suppressed in the downstreamregion, the volatility/crystallization/pressure drop problems associatedwith the use of volatile MoO₃ components in an upstream region aresubstantially attenuated or altogether avoided in the downstream region.

With reference to FIGS. 2 and 3, typically, the downstream region 130 ofdepth 3 comprises from about ½ to about ¾ of the total depth 5 of thefixed bed reactor. Preferably, the downstream region comprises fromabout ⅝ to about ¾ of the total depth 5 of the fixed bed reactor, morepreferably from about ⅝ to about ⅔ of the total depth 5 of the fixed bedreactor.

The vanadia-titania first catalyst and the metal molybdate secondcatalyst provided in the specified distribution (e.g., step S₁ ofFIG. 1) in a fixed bed reactor (e.g., in the catalyst reaction zonecomprising one or more catalytic reactor beds having the upstream regionand the downstream region) are provided in amounts, particle sizes,having surface areas and the like sufficient to oxidize an incomingreactant feed gas stream introduced into an inlet end (e.g., inlet end105 of depth 7; see FIGS. 2 and 3) to yield a product gas stream at theoutlet end (e.g., outlet end 140 of depth 8; see FIGS. 2 and 3)containing formaldehyde in a desired yield.

Referring now to step S₂ of FIG. 1, a reactant gas feed streamcomprising methanol is introduced into an inlet end (e.g., inlet end 105of depth 7; see FIGS. 2 and 3) of the fixed bed reactor. The depth 7 ofthe inlet end may be from about 1/1000 to about ½ the overall depth 5 ofthe fixed bed reactor. Alternatively, the inlet end 105 may not have anysubstantial or appreciable depth 7. In that case, inlet end 105 simplyrefers to an opening for receiving an incoming reactant gas feed stream.Typically, the reactant gas feed stream comprises methanol, air orexcess oxygen, and optionally an inert carrier gas (e.g., N₂, He, Ar, orthe like). The reactant gas feed stream enters the inlet end 105 andtravels towards the outlet end 140.

As the reactant gas feed stream approaches the upstream region, thereactant gas feed stream encounters and comes in contact with thevanadia-titania first catalyst distributed in the upstream region 110 ofthe fixed bed reactor. Accordingly, (part of step S₂ of FIG. 1)contacting the reactant gas feed stream with the vanadia-titania firstcatalyst is accomplished. The flow rate of the incoming gas feed stream,its temperature, its humidity, and other parameters are adjusted tovalues suited to oxidizing methanol to formaldehyde. The detailsnecessary to perform these adjustments are well known to those ofordinary skill in the art. However, some preferred operating parametersare provided in greater detail below.

The flow rate in terms of space velocity, where space velocity ismeasured in sec⁻¹ and defined as the sccm of gas flow per cc of catalystvolume, of the reactant gas feed stream fed into the inlet end of anexemplary fixed bed reactor ranges from about 0.1 sec⁻¹ to about 3.0sec⁻¹, preferably ranges from about 0.3 sec⁻¹ to about 2.5 sec⁻¹, morepreferably ranges from about 0.4 sec⁻¹ to about 2.2 sec⁻¹ and even morepreferably ranges from about 0.5 sec⁻¹ to about 2.0 sec⁻¹. The reactortemperature in the upstream region of an exemplary fixed bed reactortypically ranges from about 250° C. to about 450° C., preferably rangesfrom about 325° C. to about 425° C., more preferably ranges from about350° C. to about 400° C., and even more preferably ranges from about360° C. to about 380° C. The gas stream has a first molar ratio of molesof methanol to moles of oxygen from about 5:19 to about 11:9.9, andpreferably about 9.1:9.6. A second molar ratio of moles of methanol tomoles of carrier gas is from about 5:74 to about 11:83, and preferablyfrom about 9.1:83.

Upon passage of the reactant gas feed stream through the upstreamregion, a significant portion of the methanol content thereof isconverted to formaldehyde. However, some residual (yet unoxidized)methanol will be present within the partially oxidized reactant gas feedstream. At this stage through the fixed bed reactor, the reactant gasfeed stream typically also contains residual methanol, air, an inertcarrier gas (if initially provided) and other oxidation products wellknown to those skilled in the art. However, because the upstream regionof the fixed bed catalyst is essentially free of a volatile MoO₃species, the highly exothermic nature of the methanol oxidation reactionyielding formaldehyde (in the upstream region) avoids the aforementionedvolatility/sublimation/pressure drop problems associated with thepresence of volatile MoO₃ species (i.e., in an upstream region prone tohot spot formation). As it passes through and exits the upstream region,the reactant gas feed stream is partially oxidized typically containingsignificant quantities of formaldehyde.

Upon exit from the upstream region 110, the partially oxidized reactantgas feed stream typically containing formaldehyde, and residual methanol(together with other components) encounters and comes in contact withthe metal molybdate second catalyst provided in the downstream region130 of depth 3. Therein, a conventional metal molybdate second catalystis provided to oxidize the residual methanol (that may potentially bepresent in the partially oxidized reactant gas feed stream), thereby,potentially improving the formaldehyde yield.

The reactor temperature of the downstream region of an exemplary fixedbed reactor typically ranges from about 250° C. to about 450° C.,preferably ranges from about 305° C. to about 425° C., more preferablyranges from about 310° C. to about 400° C., and even more preferablyranges from about 320° C. to about 350° C.

Typically, upon passage through the downstream region, the partiallyoxidized reactant feed gas stream is now essentially fully oxidized andthe reactant gas feed stream is hereafter referred to as the product gasstream. Formaldehyde is a significant component of the product gasstream together with quantities of one or more of air, some unreactedmethanol (if any), water vapor, an inert carrier gas (if any), oxygen,and other products such as DMM (dimethoxy methane), MF (methyl formate),DME (dimethyl ether), CO, CO₂ and the like, and any other reacted andunreacted materials as may be present in the feed stream.

As the reactant gas feed stream completes passage and oxidation throughthe upstream and downstream regions, process steps S₃ and S₄ (i.e.,contacting with the vanadia-titania first and conventionaliron-molybdate second catalysts and oxidizing methanol to formaldehyde;see FIG. 1) are essentially completed. The product gas stream then exitsthe outlet end 140 of depth 8. The outlet end 140 has a depth 8 which isfrom about 1/1000 to about 1/10 the overall depth 5 of the fixed bedreactor. Often, the outlet end 140 may not have any substantial orappreciable depth 8. In that case, outlet end 140 simply refers to anopening for releasing the product gas stream.

Optionally, thereafter, the product gas stream may be recycled into theinlet, the upstream region, or the downstream region as desired forfurther oxidation. However, if the product gas stream is to be recycledinto the inlet or upstream region and possibly commingled with a freshsupply of an incoming reactant feed gas stream, it may be preferable (1)to first remove formaldehyde from the product gas stream by conventionalmeans such as distillation, condensation or absorption and the like or(2) to substantially dilute the product gas stream to minimize thepossible further oxidation of formaldehyde (in the upstream region) toits undesirable oxidation products such as CO, CO₂ or the like.Additionally, the product gas stream may be routed to another processthat utilizes the product gas stream in its state as it exits the outletend 140. Alternatively, the formaldehyde in the product gas stream maybe collected by methods known to those of ordinary skill includingdistillation, condensation, absorption or the like.

The fixed bed reactors illustrated in FIGS. 2 and 3 have an inlet end105 of depth 7, an upstream region 110 of depth 1, a downstream region130 of depth 3, and an outlet end 140 of depth 8. Each of depths 1, 3, 7and 8 may be adjusted to a value sufficient to facilitate oxidizingmethanol to formaldehyde in the desired conversion and the desiredselectivity.

Further, the exemplary fixed bed reactor of FIG. 2 has a diameter 11 andthe fixed bed reactor of FIG. 3 has a height 9 and a width 10. Each ofthe dimensions corresponding to reference numerals 9, 10, and 11 may beadjusted upwards or downwards as necessary to accommodate the desiredreactor size, the desired operating conditions, the desired conversionand selectivity.

As used herein, the term “selectivity” is determined by dividing thenumber of moles of formaldehyde formed by the number of moles ofmethanol consumed from the reactant gas feed stream times 100.Accordingly, selectivity is a percentage value. Selectivity indicatesthe percentage of formaldehyde formed as compared to the percentage ofnon-formaldehyde oxidation products of methanol such as CO, CO₂, DMM,MF, DME, etc. As used herein, the term “conversion” is determined bydividing the difference between the number of moles of methanol fed tothe fixed bed reactor in the reactant gas feed stream minus the numberof moles of methanol exiting the reactor by the total number of moles ofmethanol fed times 100. Accordingly, conversion is a percentage value.Conversion indicates the percentage of the moles of methanol that wereoxidized to formaldehyde and any other non-formaldehyde oxidationproducts of methanol. Thus, if 2 moles of methanol are fed into thefixed bed reactor (e.g., in a reactant gas feed stream) yielding 1 moleof formaldehyde and 1 mole of methanol, then selectivity would equal100% while conversion would equal 50%. Likewise, if 3 moles of methanolare fed into the fixed bed reactor (e.g., in a reactant gas feed stream)yielding 2 moles of formaldehyde and 1 mole of methanol, thenselectivity would equal 100% while conversion would equal 66 and ⅔%.

Further, the fixed bed reactors are operated at an appropriate reactortemperature, a reactor pressure and a reactant gas feed stream flow ratesufficient for oxidizing methanol to formaldehyde in the desired yield,conversion and/or selectivity. Suitable exemplary reactor temperaturesrange from about 250° C. to about 450° C. Suitable exemplary reactorpressures range from about 7 psia (i.e., about ½ atm) to about 165 psia.Suitable exemplary reactant gas space velocities range from 0.1 sec⁻¹ toabout 3.0 sec⁻¹. Other conditions suitable for oxidizing methanol toformaldehyde are used which are well known to those of ordinary skill inthe art.

The vanadia-titania first catalyst, distributed in the upstream regionof the reactor, suitable for use with this invention can be any of theknown vanadia-titania catalysts with the proviso that thevanadia-titania catalyst is essentially free of MoO₃ (or any othervolatile Mo species prone to sublimation at hot spots) in an amountsufficient to substantially impede the flow of methanol due to theaforementioned MoO₃ volatility problem. For example, the amount of MoO₃should be no more than (a) 0.1%-3% (e.g., 1%) by weight based on thetotal weight of the vanadia-titania catalyst including any catalystsupport or any other inert (or non-inert) material or (b) the moles ofV₂O₅ of the vanadia-titania catalyst. Preferably, the vanadia-titaniacatalyst, distributed in the upstream region, is free of MoO₃ and/orother similar volatile metallic oxides that poison the catalyst bed.Vanadia-titania catalysts suitable for use in conjunction with thisinvention include, but are not limited to available vanadia-titaniacatalysts where the vanadia content is less than a monolayer.

Preferably, the vanadia is primarily provided as a metal oxideoverlayer, with the oxide having a noncrystalline form. The supportedvanadia catalysts useful in the process of this invention generallycomprise a metal oxide substrate, such as titania (TiO₂), the surface ofwhich is typically modified with a layer of an oxide of vanadium (e.g.,deposited as a metal oxide overlayer). Further, the supported catalysts(e.g., V₂O₅ on TiO₂) behave differently than the unsupported bulk metaloxides (e.g., V₂O₅). Preferably, the vanadia overlaid on the titaniasupport or substrate should be sufficient to attach to the titaniasurface in an amount which does not exhibit (or does not substantiallyexhibit) properties of bulk vanadia.

Preferably, at least about 25 wt. % of the vanadia coating will be in anoncrystalline form. Typically, if the vanadia loading on the titaniasupport broadly ranges from about 0.1 wt. % to about 35 wt. % of thetotal catalyst weight then at least 25 wt. % will be in non-crystallineform. Further, titania may be employed, for example in the anataseand/or rutile forms. For example, at least about 25 wt. % (and generallyfrom about 50 wt. % to about 100 wt. %) of the titania may be in theanatase form. The above-noted wt. % values are based on the total weightof the catalyst including the weight of the support substance. Asrecognized by those skilled in the art, the titania support materialshould be sufficiently free of impurities to prevent interference withthe desired oxidizing catalytic activity. The titania may be prepared byany known conventional technique.

For example, the vanadia-titania catalysts of this invention may beprepared by impregnation techniques well-known in the art, such asincipient wetness, grafting, equilibrium adsorption, vapor deposition,thermal spreading, and the like. When using an incipient wetnessimpregnation technique, an aqueous or nonaqueous (e.g., organic)solution containing a vanadia precursor compound (together with anappropriate solvent thereof) is contacted with titania. The vanadiaprecursor (such as a salt) solution used may be aqueous or organic, withan amount of solvent sufficient to dissolve the precursor. Over time thevanadia precursor material is deposited onto the support, for example,by selective adsorption. Any excess solvent may be evaporated leavingbehind the vanadia or its precursor or salt. Other impregnationtechniques, such as vapor deposition and thermal spreading, do notrequire use of a solvent as does incipient wetness. These techniques maybe desirable in some circumstances where, for example, volatile organiccarbon (VOC) emissions are problematic. Alternatively, a suspensioncontaining the metal oxide particles may be used to spread metal oxideparticles on the substrate following the evaporation of the suspendingvehicle at a temperature from about 100° C. to about 200° C. andcalcination of the substrate and metal oxide particles at a temperaturefrom about 400° C. to about 600° C. Further, the titania used in thecatalyst of this invention may be composed of substantially porousparticles of a diameter of from about 0.4 to about 0.7 micron.Preferably the titania support has a specific surface area of at leastabout 1 m²/g (e.g., 9.5 m²/g).

One way to disperse vanadia onto a titania support is to impregnateexemplary titania spheres or powders with a solution containing avanadium compound. When impregnating a substrate with oxides, thevanadium is introduced, preferably by an aqueous solution, followed bydrying and calcining. Criteria used to select the vanadium compoundsinclude whether the compounds are soluble in the desired solvent andwhether the compounds decompose at an acceptable rate at the calcinationtemperature to give the appropriate activated vanadium oxide.Illustrative of suitable compounds of vanadium for use in conjunctionwith this invention include, but are not limited to, vanadium halides,vanadium oxyacids, vanadium oxyacid salts, vanadium oxysalts, and/orother vanadium salts. Specific examples include vanadium tribromide,vanadium dichloride, vanadium trichloride, vanadium oxychloride,vanadium oxydichloride, vanadic acid, vanadyl sulfate, vanadiumalkoxides, vanadium oxalate (which may be formed in situ by reaction ofV₂O₅ with an aqueous solution of oxalic acid), and ammoniummeta-vanadate.

The impregnation of the exemplary titania support (e.g., spheres,powders, or other shapes) with the vanadia precursor compound solutionmay be carried out, as noted above, in ways well known in the art usingeither wet or dry impregnation techniques. One convenient method is toplace the titania particles into a rotary evaporator which is equippedwith a steam jacket. An impregnating solution of a precursor compoundwhich contains an amount of the desired metal to be included in thefinished catalyst (as the metal) is added to the support particles andthe mixture is cold rolled (no steam) for a time from about 10 to about60 minutes. The cold rolling time should be sufficient to impregnate thesupport with the precursor compound solution. Water-soluble precursorcompounds are generally preferred for industrial applications because ofenvironmental concerns regarding VOC emissions. Nonetheless, when usingan organic solvent, initial heating may be done in a nitrogen atmosphereto remove any flammable solvent. Next, steam is introduced and thesolvent is evaporated. This usually takes from about 1 to about 4 hours.The impregnated support will normally be dried at temperatures rangingfrom about 50° C. to about 300° C. yielding a support with a vanadiumoverlayer thereon.

Eventually, the titania is removed from the rotary evaporator andcalcined. The evaporation and calcination is conducted in a suitableoxidizing atmosphere such as air, other oxygen source gasses, etc. at atemperature typically from about 150° to about 800° C., and morepreferably about from 400° to about 600° C., and even more preferablyfor about 1 to about 3 hours. The calcining is carried out over a periodof time sufficient to covert the precursor compound to the correspondingvanadia. As recognized by those skilled in the art, calcining conditionsneed to be adjusted to avoid undesirably reducing the catalyst surfacearea or transforming the titania via solid state reactions. As isrecognized by those skilled in the art, because some precursor compoundsare air/moisture sensitive, they are prepared under a nitrogenatmosphere or the like. The time required to calcine the composite will,of course, depend on the temperature and, in general, may broadly rangefrom about 0.5 to about 16 hours, though calcination times of less thanabout 7 hours may often be suitable. Calcination at about 450° C. forabout 2 hours has proven to be suitable for adsorbing 1 wt. % (based onthe final total catalyst weight) vanadia on the titania support. Theprecise time and temperature for calcination should be selected to avoidsubstantial crystal phase transformation of the preferred titaniumanatase form into another crystalline form, such as rutile. The anataseform is preferred to the rutile form because the former exhibits greatersurface area than the latter. As such, the former form exhibits greatercatalytic activity in comparison to that of the latter rutile form.

The vanadia supported catalysts of this invention will typically havesurface vanadia loadings from about 0.1 wt. % to about 35 wt. % (basedon the total active catalyst weight), preferably from about 1 wt. % toabout 20 wt. %, more preferably from about 1 wt. % to about 15 wt. %,and even more preferably from about 1 wt. % to about 10 wt. %.

Additional details relating to the preparation and structure of vanadiasupported catalysts suitable for use in conjunction with the inventionare well-known. For example, see Jehng et al., Applied Catalysis A, 83,179-200, (1992); Jehng and Wachs, Catalysis Today, 16, 417-426, (1993);Kim and Wachs, Journal of Catalysis, 141, 419-429, (1993); Deo et al.,Applied Catalysis A, 91, 27-42, (1992); Deo and Wachs, Journal ofCatalysis, 146, 323-334, (1994); Deo and Wachs, Journal of Catalysis,146, 335-345, (1994); Jehng et al., J. Chem. Soc. Faraday Trans., 91(5),953-961, (1995); Kim et al., Journal of Catalysis, 146, 268-277, (1994);Banares et al., Journal of Catalysis, 150, 407-420, (1994) and Jehng andWachs, Catalyst Letters, 13, 9-20, (1992).

Typically, the titania support used in accordance with the invention hasa surface area in the range from about 1 m²/g to about 150 m²/g or more(e.g., 5-15 m²/g). The titania may be used in any configuration, shapeor size which exposes its surface and any vanadia layer dispersedthereon to the gaseous stream containing methanol passed in contacttherewith. For example, titania can be employed in a particulate form ora deposited form. The deposited form includes, for example, titania (orvanadia on titania) deposited on a monolithic carrier or onto ceramicrings or pellets or the like. Typically, a vanadia on titania catalyst,is deposited on a ceramic or refractory inorganic carrier such assilicon carbide, silicon nitride, carborundum, steatite, alumina and thelike. Rings or pellets are preferred. As particles, the titania, can beformed into various shapes, including but not limited to, pills,pellets, granules, rings, spheres and the like. Alternatively, use offree particulates (e.g., unshaped titania) may be desirable when largecatalyst volumes are needed. Additionally, the active catalyst will beapplied to the inert ceramic support in an amount to provide, forexample, from about 1 wt. % to about 20 wt. % by weight, and preferablyfrom about 5 wt. % to about 15 wt. %, based on the total weight of thecatalyst including the support substrate and any carrier thereof.

Because the downstream region 130 is not as prone to formation of hotspots as is the upstream region 120, it is preferable and less expensiveto use a conventional metal molybdate catalyst in the downstream region130. Examples of metal molybdate catalysts suitable for use with theinvention include, but are not limited to Fe₂(MoO₄)₃ andFe₂(MoO₄)₃/MoO₃, other group VIII metal molybdate catalysts (e.g.,molybdates of Fe, Co, Ni, Cr, Al, Zr, Zn, Mn, or mixtures thereof). Thegroup VIII metal molybdate catalysts may contain Mo/MoO₃ in minor orlarger quantities. Preferably, the metal molybdate catalyst is a metalmolybdate/molybdenum trioxide catalyst such as Fe₂(MoO₄)₃/MoO₃. TheFe₂(MoO₄)₃/MoO₃ catalyst is preferably formed in situ during theoxidation of methanol to formaldehyde from a mixture of substantiallypure Fe₂O₃ and MoO₃ wherein an excess of MoO₃ is typically provided. Seeco-pending application designated by U.S. Provisional Ser. No.60/081,950 of Wachs et al. entitled “In Situ Formation of MetalMolybdate Catalysts,” filed Apr. 15, 1998.

The metal molybdate second catalyst typically has a surface area rangingfrom about 0.1 m²/g to about 20 m²/g or more, usually ranging from about2 m²/g to about 15 m²/g, more preferably ranging from about 4 m²/g toabout 14 m²/g, and even more preferably ranging from about 4 m²/g toabout 12 m²/g.

Having described the invention, the following illustrative examples areprovided. These examples are illustrative of preferred aspects of theinvention and are not intended to limit the scope of the invention. Allpatents, publications and any other references cited herein areincorporated by reference herein in their entirety, respectively. Inthat regard, related provisional applications (1) “Dual Catalyst BedReactor for Methanol Oxidation” designated by Ser. No. 60/232,426, filedon even date and (2) “Metal Molybdate/Iron-Molybdate Dual Catalyst BedSystem and Process Using the Same for Methanol Oxidation toFormaldehyde” designated Ser. No. 60/232,628 filed on even date areincorporated herein by reference in their entirety.

EXAMPLE 1

The oxidation of methanol over the catalyst distribution of the claimedinvention was examined in a fixed-bed reactor: 1″ outer diameter, 14Birmingham Wire Gauge, 5′ length steel tube and catalyst fillage is18-48″ (e.g., 36″: the sum of 24″ of Perstorp KH-44 Fe—Mo in thedownstream region with 12″ of V₂O₅/TiO₂ in the upstream region) with0″-24″ inert rings (e.g., inert ceramic rings from Perstorp Polyols,Inc. of Toledo, Ohio) on the bottom (i.e., between the catalyst bed andthe outlet end) and 0″-24″ inert rings on the top (i.e., between theinlet end and the catalyst bed).

Each catalytic test consisted of a gas stream of CH₃OH/O₂/He with amethanol feed concentration of 9.1% and a total flow rate of 28.3 slpm(standard liters per minute). Methanol, oxygen, and nitrogen werepresent in a molar ratio of 9.1/9.6/81.3.

In order to examine the full effect of temperature on methanoloxidation, the reactor temperature was varied from about 260° C. toabout 300° C. under recycle conditions. The reaction products wereanalyzed with a gas chromatograph: MTI Q30H with one 14 meter OV-1column, one 8 meter Stabilwax column and one 10 meter Molsieve 5A columnobtained from MTI of Fremont, Calif. The test results for a catalyst bedwith 12″ of V₂O₅/TiO₂ in the upstream region followed by 24″ of PerstorpKH-44 Fe—Mo in the downstream region are presented in Table 1 below.

The catalyst of the claimed invention yielded about a 97% conversion anda selectivity (formaldehyde) of 87% at 290° C. in the laboratory.

TABLE 1 Methanol Conversion and Product Selectivity ConversionSelectivity (%) T (° C.) (%) HCHO DME MF DMM CO CO₂ 260 92.7 90.7 0.750.07 0 6.83 0.07 270 94.8 90.6 0.45 0.03 0 7.63 0.16 275 95.7 87.5 0.350.03 0 11.05 0.18 280 95.6 88.1 0,33 0.05 0 10.48 0.22 280 96.5 84.00.12 0.02 0 14.93 0.28 290 97.1 86.6 0.21 0.02 0 12.42 0.21 300 98.078.7 0.07 0.02 0 20.50 0.30 HCHO indicates the selectivity forformaldehyde. Likewise, DME, MF, and DMM indicate the selectivity fordimethyl ether, methyl formate, and dimethoxy methane, respectively.

EXAMPLE 2

Instead of the reactor indicated in EXAMPLE 1 above, one may use thefollowing reactor which is a self-contained formaldehyde manufacturinglaboratory:

-   -   1. Liquid methanol storage, metering and vaporization equipment.    -   2. Process gas compression and metering equipment.    -   3. A reactor unit comprised of:        -   a. A one inch OD, 60 inch long, 14 BWG, carbon steel tube            surrounded by:            -   1) A circulating, temperature controlled, bath of a heat                transfer oil with an atmospheric boiling point of 257°                C.            -   2) A vapor pressure measurement and control system for                the heat transfer oil.            -   3) A stainless steel tube thermowell of diameter of                0.125 inch extending axially for the length of the tube,                containing a wire thermocouple capable of being                positioned at any point within the length of the                thermowell.    -   4. A catalyst bed installed in the reactor tube void space of        0.834 inch diameter, which may vary in depth from essentially        zero inch to 48 inches and may be comprised of one or more        subdivisions of catalyst types, mixtures, or inert materials.    -   5. Formaldehyde separation equipment commonly known as an        “Absorber” comprised of four sections of packing and the        required handling equipment for circulation and cooling.    -   6. Piping and equipment necessary for operation of the catalyst        bed in a recycle mode so as to allow the oxygen level of the        process gas to be controlled at less than atmospheric levels.    -   7. Oxygen measuring and control equipment for maintaining        precise control of oxygen in the catalyst bed.    -   8. Instrumentation and equipment to control the process gas        pressure during the formaldehyde manufacturing process.

The above-identified reactor may be used with a catalyst bed having 12″of V₂O₅/TiO₂ in the upstream region followed by 24″ of Perstorp KH-44Fe—Mo in the downstream region thereof.

It will be understood that the claims below are intended to cover allchanges and modifications of the examples and preferred embodiments ofthe invention herein chosen for the purpose of illustration which do notconstitute departures from the spirit and scope of the invention.

1. A fixed bed reactor for oxidizing methanol in a reactant feed streamto formaldehyde, said fixed bed reactor comprising an inlet end, adepth, a length, a width, an upstream region, a downstream region, andan outlet end wherein said fixed bed reactor comprises: (a) avanadia-titania first catalyst in said upstream region wherein saidvanadia-titania first catalyst is substantially free of MoO₃; and (b) ametal molybdate second catalyst in said downstream region wherein saidmetal molybdate second catalyst initially is free or substantially freeof V₂₀₅O₅ and wherein said metal molybdate second catalyst is selectedfrom the group consisting of molybdates of iron (Fe), cobalt (Co),nickel (Ni), chromium (Cr), aluminum (Al), zirconium (Zr), zinc (Zn),manganese (Mn) and mixtures thereof.
 2. The fixed bed reactor of claim1, wherein said metal molybdate second catalyst is Fe₂O₃/MoO₃.
 3. Thefixed bed reactor according to claim 1, wherein said upstream region isfrom about a first fourth to about a first half of said depth of saidfixed bed reactor nearest said inlet end, and said downstream region isfrom about three-fourths to about one-half of said depth of said fixedbed reactor nearest said outlet end.
 4. The fixed bed reactor accordingto claim 1, wherein said upstream region is from about a first third toabout a first half of said depth of said fixed bed reactor nearest saidinlet end, and said downstream region is from about two-thirds to aboutone-half of said depth of said fixed bed reactor nearest said outletend.
 5. The fixed bed reactor according to claim 1, wherein saidupstream region is about a first third of said depth of said fixed bedreactor nearest said inlet end, and said downstream region is abouttwo-thirds of said depth of said fixed bed reactor nearest said outletend.
 6. The fixed bed reactor according to claim 1, wherein a depthratio of said upstream region to said downstream region of said fixedbed reactor is about 1:2.
 7. The fixed bed reactor of claim 1 whereinthe metal molybdate second catalyst is a metal molybdate/molybdenumtrioxide catalyst.